Which structure helps prevent over-expansion of the cell when water enters?

The correct answer is the cell wall, as it serves a critical function in plant cells and some prokaryotic cells by providing structural support and rigidity. When water enters the cell, it can cause the cell to swell due to osmosis. The cell wall acts as a protective barrier that prevents excessive expansion by exerting pressure against the internal turgor pressure that builds up as the cell fills with water. This turgor pressure is essential for maintaining the cell's shape and ensuring its integrity, allowing plants to maintain their upright position.

While the cell membrane is involved in regulating the movement of water and solutes, it does not provide the same level of rigidity and support that the cell wall does. The vacuole, although it acts as a storage organelle and can contribute to turgor pressure, is not the primary structure responsible for preventing over-expansion. The cytoskeleton plays a role in maintaining cell shape and assisting with cellular movement but does not function primarily to prevent over-expansion from water influx. Therefore, the cell wall is the key structure that prevents cells from bursting when exposed to excess water.